Poly(trimethylene terephthalate) fiber (hereinafter poly(trimethylene terephthalate) is referred to as PTT) is surprisingly excellent in soft feeling, drapability, stretchability, low-temperature dyeability, weather resistance or others, and has numerous characteristics superior to the existing synthetic fibers such as poly(ethylene terephthalate) fiber (hereinafter poly(ethylene terephthalate) is referred to as PET) or nylon 6 fiber.
The present applicant has overcome numerous difficulties relating to the polymerization or spinning of PTT, the processing of PTT fiber, the merchandising thereof or the like and has recently marketed PTT fiber for the first time in the world (under the trade name “Solo”).
PTT is obtained by the polycondensation of terephthalic acid or lower alcohol ester of terephthalic acid with 1,3-propanediol (also called as trimethylene glycol; hereinafter 1,3-propanediol is referred to as PDO).
Elementary processes constituting the polycondensation reaction of PTT mainly comprise the following reactions. The forward reaction is a chain propagation caused by the de-PDO (PDO removal reaction) of two terminal hydroxyl groups (see the following formula (a)). The backward reaction is a reaction in which an ester portion is decomposed with undischarged PDO (that is, the backward reaction of the formula (a)) and the thermal decomposition of the ester portion (see the following formula (b)). In this regard, in the formula (a), k1 is a reaction rate constant in the rightward reaction and k2 is a reaction rate constant in the leftward reaction. Also, in the formula (b), kd is a reaction rate constant in the rightward reaction. 
As PTT is more easily thermal-decomposable than PET or polybutylene terephthalate (hereinafter referred to as PBT) having a skeleton similar to that of PTT (in other words, as PTT has a larger kd), it is difficult to increase the molecular weight solely by the melting polymerization. Accordingly, to obtain PTT having a high molecular weight, both the melting polymerization and the solid-state polymerization are usually used, in which prepolymer having a low molecular weight is first produced by the melting polymerization and once cooled and solidified, and then, the polycondensation is carried out at a temperature of the melting point of the prepolymer or lower.
However, there are various problems in the melting polymerization or the solid-state polymerization of PTT due to the polymer properties inherent to PTT.
First, PTT is liable to be thermally decomposed in the melting process. As PTT has a larger kd in the above formula (b), the viscosity thereof is liable to lower in the melting state. Molecular terminal carboxyl groups or molecular terminal aryl groups created by the thermal decomposition in the melting state further accelerate the thermal decomposition to cause the deterioration of the degree of whiteness or the antioxidation stability of PTT. Thus, in the production process of PTT, it is necessary to suppress the thermal decomposition as much as possible for the purpose of obtaining high-quality PTT. Such a problem, however, has not been satisfactorily solved in the prior art.
Second, PTT pellets are easily cracked or powdered. For example, when the pellets are rubbed against each other during the solid-state polymerization, drying or transportation, they are relatively easily cracked or powdered. Particularly, if the PTT pellets are cracked or powdered during the solid-state polymerization, various problems are generated in the spinning, film-forming or molding process, such as the generation of yarn breakage, fluff or fish eye.
That is, since the powdery PTT has a large surface area, if it is mixed with the pellets and subjected to the solid-state polymerization, the discharge of PDO in the above formula (a) is excessively effectively carried out, which results in a higher molecular weight in comparison with the pellet-shaped PTT and thus in an extraordinary high melting viscosity. Accordingly, if the PTT obtained by the solid-state polymerization is used for the melt-molding process, the powdery material thus having a high molecular weight is not completely melted during the melt-molding process to cause the irregular melting state of polymer, resulting in yarn breakage or fluff in the spinning process. Also, the powdery material adheres to an interior wall of a vessel for the solid-state polymerization and dwells there for a long period to cause the thermal deterioration or color development. If this material is discharged, the color or the antioxidation stability of the melt-mold product may be degraded. To avoid such problems, cracked or powdered pellets may be removed prior to being melt-molded. However, this needs a removal process and, if a large amount of such pellets is generated, the loss of raw material becomes significant to push up the production cost.
The two above-mentioned problems hardly occur in PET and PBT although they have a chemical structure similar to that of PTT. This is because these polymers have a constant of thermal decomposition rate corresponding to a kd much smaller than that of PTT as well as an amount of powdery material generated when the pellets of such polymers are rubbed against each other is very small. In other words, the above-mentioned problems are peculiar to PTT and, therefore, it is very difficult to think of a solution for this problems from the descriptions in known documents relating to PET and PBT. Also, there is neither disclosure nor suggestion for solving these problems in known documents relating to the polymerization of PTT.
For example, in Japanese Unexamined Patent Publication No. 2000-159875, a method is proposed, in which polymer having a small amount of terminal vinyl groups obtained by the melt-polycondensation while using a mixed catalyst of Ti and Mg in a certain state is subjected to the solid-state polymerization in an inert gas atmosphere to result in high-quality PTT. However, according to this method, the resultant pellet is dull in hue and low in L* value in a range from approximately 60 to 70 because Mg is used as a catalyst. There is nothing about the generation of powdery material or, of course, a suggestion of a solution thereto.
WO97/23543 discloses a method, in which non-pelletized PTT having a low polymerization degree is melted and dropped on a hot plate, so that it is crystallized at a temperature in a range from 60 to 190° C. to form solid PTT having crystallites of a size of 18 nm or more, which is then subjected to the solid-state polymerization. PTT obtained by this method, however, has a rough surface and is liable to generate powder when rubbed. Also, there is no disclosure of a technique for improving the color tone and the antioxidation stability in this description.
In the US patent 2001/0056172-A1, a method for the solid-state polymerization of PTT pellets is described. In this method, however, the solid-state polymerization technique generally used for obtaining PET is merely applied to PTT, and there is neither the recognition of problems peculiar to PTT, such as the deterioration of whiteness and antioxidation stability or the generation of powdery material, nor the solution thereof.
In Embodiment 8 of WO98/23662, a method is disclosed, in which the solid-state polymerization is carried out after PTT terminal-sealed with hindered phenolic type stabilizer has been pelletized. Also, in Embodiment 8 of WO99/11709, a method is disclosed, in which the solid-state polymerization is carried out after PTT containing phosphor type stabilizer has been pelletized. In these methods, however, there is neither the recognition of problems such as the deterioration of antioxidation stability or the generation of powdery material, nor a solution thereto.